1. Field of the Invention
This invention relates to a screw for molding thermoplastic resin, used in an extrusion equipment.
2. Description of the Prior Art
Various molded parts such as a sheet, film, container are obtainable from thermoplastic resins through extrusion-, blow-, injection-molding and the like. In each of the above-described fields of molding, construction of the screw in the extrusion equipment is an important factor for performing heating, melting and kneading of the resin.
The conventional most common screw having its main object to satisfactorily perform kneading of a molten resin by increasing a compression ratio is typified by a full flight metering screw successively provided from a resin feed side to a resin extrusion side with a feed section 1, compression section 2 and metering section 3 as shown in FIG. 1, for example. As a consequence, although kneading of the resin is carried out satisfactorily, the compression ratio is high and high shearing is accompanied, thus presenting some disadvantages. More specifically, when the compression ratio is high and high shearing is accompanied, pressure of a resin tends to fluctuate during high-speed extrusion molding, so that molded parts of high quality cannot be consistently obtained. Furthermore, satisfactory kneading during the high-speed extrusion molding leads to that the resin in the proximity of the forward end portion of the screw is subjected to high shear stress, this high shear stress brings about mechanical heating to raise the temperature of the resin beyond necessity, making the resin low in density, the resin is deteriorated in material quality to lower the physical properties of the molded parts. It becomes difficult in takeup and sizing during extrusion molding and driveling tends to occur during injection molding.
Furthermore, during high-speed extrusion molding, a large driving force is required for driving the screw, and this driving force causes the temperature of the resin to rise. As a consequence, in order to prevent the overheat of this driving energy, and further, the overheat of the temperature of the resin, it becomes necessary to cool the heating from outside, whereby a great problem is presented from the viewpoint of energy saving and the viewpoint of requiring equipment for cooling, so that solution of this problem is demanded.
Particularly, these problems are highly true of the case of thermoplastic resins such as polypropylene, linear low density polyethylene, polycarbonate and polystylene out of various thermoplastic resins, said thermoplastic resins showing newtonian shear properties (properties showing relatively high melt viscosity under a high rate of shear) caused by such a factor that the molecular-weight distribution thereof is relatively narrow.
As a result, there have been proposed various screws each having a construction wherein the compression ratio is made relatively small, which is made less than 1 to control generation of shearing heat as the case may be. However, in each of these cases, kneading becomes unsatisfactory, whereby temperature of the resin is ununiformized and diffusion of various additives becomes unsatisfactory, so that it is difficult to obtain satisfactory molded parts and the scope of utilization thereof is limited. Furthermore, as one to improve the above-described respects, there has been proposed a construction of two stage screws. However, stress relaxation is not satisfactory, whereby molded parts excellent in surface characteristics and transparence are not obtainable.
Now, to obviate the disadvantages of the prior art, the present applicant has proposed a novel technique wherein a compression section of a screw is turned into a relaxed compression section having a relatively small compression ratio (Japanese Patent Application No. 266398/1984), and the present invention has the same object as described above and has further improved the aforesaid invention.